Long-Range Volcanic Ash Transport and Fallout During the 2008 Eruption of Chaitг©N Volcano, Chile

Physics and Chemistry of the Earth xxx (2011) xxx–xxx

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Physics and Chemistry of the Earth

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Long-range volcanic ash transport and fallout during the 2008 eruption of Chaitén volcano, Chile ⇑ Adam J. Durant a,b,c, , Gustavo Villarosa d, William I. Rose b, Pierre Delmelle e, Alfred J. Prata a, José G. Viramonte f a Norwegian Institute for Air Research, P.O. Box 100, NO-2027 Kjeller, Norway b Geological and Mining Engineering and Sciences, Michigan Technological University, USA c Centre for Atmospheric Science, Department of Chemistry, University of Cambridge, Lensﬁeld Road, Cambridge, CB2 1EW, UK d INIBIOMA, CONICET – Universidad Nacional del Comahue, Quintral 1250, 8400 Bariloche, Río Negro, Argentina e Environment Department, University of York, Heslington, York, UK f Universidad Nacional de Salta – CONICET – INENCO-GEONORTE, Av. Bolivia 5150, 4400 Salta, Argentina article info abstract

Article history: The May 2008 eruption of Chaitén volcano, Chile, provided a rare opportunity to measure the long-range Available online xxxx transport of volcanic emissions and characteristics of a widely-dispersed terrestrial ash deposit. Airborne ash mass, quantified using thermal infrared satellite remote sensing, ranged between 0.2 and 0.4 Tg dur- Keywords: ing the period 3–7 May 2008. A high level of spatiotemporal correspondence was observed between Volcanic ash cloud trajectories and changes in surface reflectivity, which was inferred to indicate ash deposition. Chaitén The evolution of the deposit was mapped for the first time using satellite-based observations of surface Fallout reflectivity. Satellite remote sensing The distal (>80 km) ash deposit was poorly sorted and fine grained, and mean particle size varied very Aggregation Ocean fertilisation little beyond a distance >300 km. There were three particle size subpopulations in fallout at distances >300 km which mirror those identified in fallout from the 18 May 1980 eruption of Mount St. Helens, known to have a high propensity for aggregation. Discrete temporal sampling and characterisation of fallout demonstrated contributions from specific eruptive phases. Samples collected at the time of deposition were compared to bulk samples collected months after deposition and provided some evidence for winnowing. Experimentally-derived ash leachates had near-neutral pH values and charge balance which indicates minimal quantities of adsorbed acids. X-ray Photoelectron Spectroscopy (XPS) analyses revealed surface enrichments in Ca, Na and Fe and the presence of coatings of mixed Ca-, Na- and Fe-rich salts on ash particles prior to deposition. Low S:Cl ratios in leachates indicate that the eruption had a low S content, and high Cl:F ratios imply gas–ash interaction within a Cl-rich environment. We estimate that ash fallout had potential to scavenge 42% of total S released into the atmosphere prior to deposition. XPS analyses also revealed ash particle surfaces were strongly enriched in Fe (in contrast to the results from bulk leachate analyses), which suggests that Chaitén ash fallout over oceans had potential to influence productivity in high-nutrient, low-chlorophyl regions of the oceans. Therefore ash particle surface geochemical analysis

1. Introduction and long-range transport of volcanic emissions in the lower stratosphere, and characterise associated fallout over a largely terrestrial Ash fallout from explosive volcanic eruptions is rarely deposited setting. Clouds from the eruption deposited ash over Chile and in entirety on terrestrial surfaces and some fraction inevitably ends Argentina and formed the ﬁrst continental-scale ash deposit since up deposited in the oceans. The May 2008 eruption of Chaitén vol- the eruptions of Mount St. Helens in 1980 and Cerro Hudson in cano, Chile, provided a rare opportunity to measure the dispersion 1991. Adsorbed soluble (ephemeral and mobile) chemical compo- nents related to eruption of volcanic gases and associated incrusta-

⇑ Corresponding author at: Climate and Atmosphere Department, Norsk Institutt tions may generate a range of direct and indirect biological, for Luftforskning, Instituttveien 18, P.O. Box 100, NO-2027 Kjeller, Norway. chemical and physical effects following deposition into terrestrial E-mail address: [email protected] (A.J. Durant). and aquatic ecosystems (Óskarsson, 1980; Witham et al., 2005).

The type and magnitude of these impacts depend on various fac- a large column that reached 17–20 km MSL (observed by G. Villa- tors, including the ash chemical and physical properties, deposit rosa at 14:15 ART/17:15 UTC) from the surface at a location near thickness and residence time, and sensitivity of the local environ- Esquel, Argentina (42.9°S 71.317°W), 124 km from the volcano). ment. Although a large number of eruptions have been investi- Low level ash emission continued to sustain an ash column with gated (Witham et al., 2005), this is the first such investigation of height of <10 km MSL between 3 and 5 May (Watt et al., 2009). a rhyolitic ash-fall and the environmental effects of associated rhy- Phase 3 of the eruption initiated on 6 May 2008 and generated a olitic ash leachates. column that was estimated to have reached in excess of 30 km In this study, we analyse: (1) changes in airborne ash mass in (GOES observations at 1200 UT on 6 May 2008) (Carn et al., the volcanic clouds produced by the eruption as a function of time 2009). Airborne ash and/or ice-coated ash in the dispersed cloud from satellite observations and related to observed fallout; (2) sed- from this phase was observed using CALIOP at a height of imentological characteristics of ash fallout collected at or very near 16 km MSL the following day between 41 and 42°S(Thomason the time of deposition, i.e., pristine and unaffected by hydrological and Pitts, 2008). Phase 4 began on 8 May 2008 with the eruption or aeolian processes post-deposition; and (3) the composition of of an ash column to 20–22 km MSL. Volcanic aerosol from this chemical leachates released through deposition of rhyolitic ash phase was observed using CALIOP at a height of 13 km MSL the fol- fallout in aqueous environments. lowing day (Carn et al., 2009; Thomason and Pitts, 2008). Emission continued into May and June at low intensities with ash columns 1.1. The May 2008 eruption of Chaitén, Chile generally reaching mid- to upper tropospheric heights. By July activity had diminished and occasional ash eruptions generated The eruption chronology reported here combines previous ac- plumes that reached between <2 and 3 km (Simkin and Siebert, counts (Carn et al., 2009; Folch et al., 2008; Lara, 2009; Watt 2002). et al., 2009) and additional surface and remote sensing observa- Ash fallout from the eruption was widespread and impacted tions of the eruption (Table 1). The May 2008 eruption of Chaitén large regions of Chile and Argentina (Watt et al., 2009). Ash emis- was unexpected as the previous documented eruption at the vol- sions from Phase 1 were dispersed to the southeast over Argentina, cano occurred nearly 10,000 years ago (Naranjo and Stern, 2004) although an early emissive pulse on 2 May was transported to the though new tephrochronological data from surface and lake re- east and northeast of Chaitén volcano, and ash fallout was cords in Argentina suggest that there were other important explo- observed at Esquel, Argentina (42.907°S 71.308°W), 110 km sive events during the Holocene (Iglesias et al., 2011). Seismic away from Chaitén volcano and Epuyén (42.233°S 71.350°W) activity increased on 30 April 2008 with swarms of volcano-tec- 130 km to the northeast (Fig. 5). Ash fallout was observed at tonic earthquakes ranging in magnitude from 3 to 5 (Carn et al., Comodoro Rivadavia on the Argentine coast (45.87°S 67.48°W) 2009). Between 1 May and 2 May 2008 the frequency of events in- over 500 km to the SE approximately 16 h after the eruption began. creased and reached a maximum of 15–20 per hour prior to the A sheared portion of the cloud was transported to the NW as far as first major eruption (Phase 1) at 08:00 UTC on 2 May (Table 1) Bariloche, Argentina (41.16°S 71.34°W), about 215 km NNE of Cha- that generated a column in excess of 21 km above mean sea level itén in the evening of 2 May 2008, although no ash-fall was re- (MSL) (Lara, 2009). Geostationary Operational Environmental Sa- ported at the time. tellite (GOES) thermal infrared imagery indicated a cloud height Emissions from Phases 2–4 were dispersed first to the southeast of 12 km MSL at 0800 UT on 2 May 2008 (Carn et al., 2009) for towards Comodoro Rivadavia on 3 May, and then east towards the the dispersed part of the cloud which was corroborated a day later Argentine coastal settlements of Trelew (43.249°S 65.307°W) and by cloud height estimates from a space-borne lidar (CALIOP; Puerto Madryn (42.754°S 65.049°W) where ash fallout was ob- Thomason and Pitts, 2008). After a period of quiescence on the served on 5 May. Ash emissions from Phase 3 of the eruption (Table morning of 3 May 2008, Phase 2 of the eruption was initiated by 1) were carried to the northeast on 6 May and reached Neuquén,

Table 1 Eruption chronology for the May 2008 eruption of Chaitén volcano, Chile.

Phase Date Time (UTC) Observation Cloud height (location) Measurement source Reference 1 2 May 2008 08:00–14:00 Initial explosive ash column >21 km (Chaitén volcano) Visual observation (PIREP) (Carn et al., 2009; Folch et al., 2008) Morning Early column activity 10.7–16.8 km Visual observation (SGVP) (Folch et al., 2008) Stratospheric cloud height 12 km GOES imagery (Carn et al., 2009) 2 3 May 2008 18:15 Explosive ash column 17.4–19.6 km Visual observation (clinometer) G. Villarosa 3–5 May 2008 Sustained explosive ash emission <10 km (Chaitén volcano) (Watt et al., 2009) 3–4 May 2008 Fine volcanic ash and/or ice crystals 12 km (30°S) CALIOP/OMI (Carn et al., 2009) 3 6 May 2008 12:00* Initial explosive ash column 30 km (Chaitén volcano) (Carn et al., 2009) 6 May 2008 13:30* Initial explosive ash column 30 km (Chaitén volcano) Visual observation (ONEMI) (Folch et al., 2008) 20:00 Increase in eruption intensity (Folch et al., 2008) 7 May 2008 Eruption column 7–10 km CALIOP (Folch et al., 2008) 7 May 2010 Ash / ice-coated ash cloud 16 km (41–42°S) (Thomason and Pitts, 2008) 4 8 May 2008 03:30 Initial explosive ash eruption 20–22 km (Chaitén volcano) GOES imagery (Carn et al., 2009) 8 May 2008 Airborne ash 3–10 km (Buenos Aires) (Folch et al., 2008) 9 May 2008 Volcanic aerosol 13 km CALIOP (Carn et al., 2009)

CALIOP – Cloud-Aerosol Lidar with Orthogonal Polarisation (on Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations platform). GOES – Geostationary Operational Environmental Satellite. OMI – Ozone Monitoring Instrument. ONEMI – Oficina Nacional de Emergencia del Ministerio del Interior (National Office of Emergency of the Interior Ministry, Chile). PIREP – Pilot report. SGVP – Smithsonian Global Volcanism Program. * These reports conflict on the time of onset of the initial activity of Phase 3.

Argentina (38.949°S 68.066°W), approximately 575 km northeast as a function of time. The assumption underlying the analysis is of Chaitén, some 8–14 h after the initial eruption. On 7 May, the simple: ash fallout modifies surface albedo and MODIS measures first ash fall was observed at Bariloche and later ash fallout was ob- the directional surface reflectance at several wavelengths. We used served at Bahía Blanca (38.711°S 62.268°W) and Mar Del Plata a combination of MODIS channels, and through trial-and-error (37.980°S 57.590°W), approximately 1000 km and 1400 km, found a subset that was best able to indentify changes in reflec- respectively, from Chaitén. Ash emissions from Phase 4 (Table 1) tances associated with ash fallout. The directional reflectances were carried to the north via Neuquén, and then to the northwest were normalised by geometrical factors that depend on the solar as far as Concepcion, Chile (36.81°S 73.03°W) over 650 km to the and satellite viewing angles. No attempt was made to relate the north northwest by 1600 UTC on 8 May 2008. Ash fallout after this changes in reflectance to the accumulation of ash on the ground, time was minimal other than a later explosive pulse on February instead each image was analysed to estimate the area difference 19 2010 that produced a deposit that extended over 200 km south- in area from a reference image, taken as containing the least clouds east from Chaitén volcano. over the preceding 2 weeks prior to the eruption. Difficulties were There were three stratigraphic units (a, A-M, b) identified in found in estimating these changes in regions of rapid topographic deposits within 25 km of the volcano formed from activity be- change (e.g. the eastern side of the southern Andes) and these tween the periods 1–2, 3–5, and 6 May, respectively (Alfano areas were masked out. et al., 2010). Unit a was mapped to the northeast, A–M to the southeast and b again to the northeast. The distal (>100 km) 2.2. Deposit characterisation deposit from the May 2008 eruption comprised 1 Â 1011 kg ash (dense rock equivalent, DRE 0.07 km3) which was deposited An ash collection network was set up in Argentina in coopera- over 2 Â 105 km2 (Watt et al., 2009). The erupted tephra consisted tion with national, provincial and municipal agencies. Fallout over of ash (80 wt.%), pumice lapilli and bombs (17 wt.%), and the Andean region was collected using traps and sampled specific obsidian fragments (3 wt.%). Compositional analysis of pumice ash fall events related to discrete phases of the eruption (or shortly indicated that a crystal-poor rhyolitic (73–76 wt.% silica content) after every ash fall event) over a vast area within 250 km of the vol- À magma with 1.3–2.3 wt.% water (H2O and OH ) was erupted cano (Figs. 5 and 6; Table 2). In some cases samples were collected during the May 2008 event (Alfano et al., 2010; Castro and within hours and up to 5 days after deposition by non-specialists Dingwell, 2009). Satellite observations (Ozone Monitoring Instru- (including engineers and park rangers). These samples may have ment) indicated the collective mass of SO2 released from all been exposed to light precipitation and different degrees of win- phases of the eruption only totalled 10 kT (Carn et al., 2009). nowing (Table 2). The quantity of respirable (<4 lm) particles in ash fallout from the eruption ranged from 8.8 to 11.9 vol.% and contained 2.2– 2.3. Particle size analysis 7.4 wt.% cristabolite, which posed a low human health hazard (C. Horwell, personal communication 2011). Approximately Particle size was measured through application of low angle la- 400 km2 of native evergreen forest were damaged by the 2008 ser diffraction particle size analysis (Blott et al., 2004) using a Mal- eruption of Chaitén (Pallister et al., 2010) and in areas beyond vern Instruments Mastersizer 2000. In this technique, the ash the blast-affected zone, tephra fallout stripped foliage and sample is suspended in water, circulated through a measurement branches from the forest canopy. There were numerous impacts cell, and light at different wavelengths (red and blue) is passed on agriculture and infrastructure (Wilson et al., 2009a,b). through the dispersed sample and the diffraction pattern is measured. Particle size is determined through application of a particle scattering model (Lorenz-Mie) and assumptions are made regard- 2. Methods ing particle refractive index and shape (spherical particle assumption). In this study we assumed ash samples had a rhyolitic 2.1. Satellite data analysis composition and used a refractive index of 1.5 (Horwell, 2007). The instrument returns distributions of volume-based frequency MODIS infrared and visible radiances were analysed to assess between size classes spanning 0.02–2000 lm. the amount of fine ash suspended in the atmosphere and to esti- Particle size subpopulations were identified following the mate the area of land covered by ash fallout from the Chaitén erup- method outlined in Wohletz et al. (1989). The measured particle tion. The infrared data were used to retrieve particle effective radii size distribution of a sample taken from a discrete location may

(reff, weighted mean of the ash particle size distribution), infrared be deconvolved into a series of subpopulations that initially result optical depth and mass loadings (g mÀ2) using a combined radiative from fragmentation at the vent (primary magma rupture and sec- transfer and ash microphysical model (Prata and Grant, 2001; Wen ondary fragmentation in the conduit), and are subsequently mod- and Rose, 1994). The ash microphysical component assumes air- ified by cloud processing during transport. Microphysical processes borne ash is composed of spherical particles with a modified gam- (e.g., hydrometeor formation) and gravitational settling result in ma particle size distribution and a rhyolitic composition. The size selective sorting during transport and sedimentation. Identifi- wavelength dependent refractive indices were taken from Pollack cation of particle size subpopulations may be mapped out to et al. (1973); this composition improves results over assumed understand the propensity of various processes that modify the andesitic or basaltic refractive indices (Gangale et al., 2010). The to- initial particle size distribution. tal mass of fine ash in each MODIS image was calculated by finding the sum of each pixel mass loading (nominally 1 km2) multiplied by 2.4. Leachate analysis the area of the cloud. The methodology may generate underestima- tions mostly due to the presence of highly opaque clouds, and over- In order to assess the release of chemical species following estimations due to uncertainty in the underlying assumptions, deposition of the ash material in aqueous environments, extrac- highly transparent clouds and noisy data. This leads to a typical er- tions were carried out on samples through immersion and agita- ror in cloud mass estimates of around 20%. tion in deionized water (ash-to-water ratio of 1:25) for 45 min in The infrared-derived mass loadings were complemented by an acid-washed polyethylene tube. The slurry was centrifuged analyses of visible light reflected from the surface, also using and the pH of the supernatant leachate was measured prior to MODIS data, to estimate the surface area impacted by ash fallout filtration through 0.2 lm cellulose acetate membrane filters. The

Table 2 Summary of samples collected in this study.

Sample ID Collection date Collection time Eruption date Latitude Longitude Distance (km) Comments

CH0805_1 4 May 2008 2–4 May 2008 À43.176531° À71.550786° 97.4 CH0805_2 9 May 2008 2–4 May 2008 À43.123936° À71.552567° 95.4 CH0805_3 6 May 2008 2–4 May 2008 À43.069028° À70.940778° 142.2 CH0805_4 2 May 2008 2 May 2008 À42.911356° À71.322956° 109.1 No rain CH0805_5 4 May 2008 3–4 May 2008 À42.911356° À71.322956° 109.1 No rain CH0805_6 6 May 2008 5–6 May 2008 À42.911356° À71.322956° 109.1 No rain CH0805_7 9 May 2008 7 May 2008 À42.911356° À71.322956° 109.1 Direct fallout sampled CH0805_8 8 May 2008 2–7 May 2008 À42.025883° À70.804133° 177.4 CH0805_9 9 May 2008 3–4 May 2008 À43.492419° À70.810494° 166.7 CH0805_10 7 May 2008 7 May 2008 À42.081200° À71.548117° 124.7 Direct fallout sampled CH0805_11 7 May 2008 3–4 May 2008 À43.250000° À71.333333° 116.9 CH0805_12 7 May 2008 3–4 May 2008? À43.134267° À71.434117° 104.6 CH0805_13 7 May 2008 2–7 May 2008 À43.088569° À71.505208° 97.6 Direct fallout sampled CH0805_14 7–8 May 2008 20:00–09:00 7 May 2008 À41.124472° À71.384333° 218.6 Direct fallout sampled CH0805_15 7 May 2008 3–7 May 2008? À43.134417° À71.611444° 91.2 CH0805_16 7 May 2008 3–7 May 2008? À43.136300° À71.438833° 104.5 CH0805_17 6 May 2008 3–4 May 2008 À43.785694° À70.934194° 174.8 CH0805_18 6 May 2008 3–4 May 2008 -44.059361° À70.394417° 227.9 CH0805_19 6 May 2008 11:30–13:30 6 May 2008 À42.209683° À71.405239° 124.3 Direct fallout CH0805_20 5 May 2008 00:00–9:00 5 May 2008 À42.911356° À71.322956° 109.1 Direct fallout sampled at Esquel CH0805_21 4 May 2008 15:05 3–4 May 2008 À43.196167° À71.607617° 94.2 Some light rain CH0805_22 4 May 2008 15:24 3–4 May 2008 À43.175450° À71.691050° 86.9 Some light rain CH0805_23 4 May 2008 16:18 3–4 May 2008 À43.173600° À71.749617° 82.6 Some light rain CH0805_24 3 May 2008 13:40 2–3 May 2008 À42.626350° À71.065850° 132.1 No rain CH0805_25 3 May 2008 18:14 3 May 2008 À43.136300° À71.438833° 104.9 No rain CH0805_26 6 May 2008 6 May 2009 À41.940000° À71.493611° 138.6 No rain CH0805_27 7 May 2008 7 May 2009 À41.940000° À71.493611° 138.6 No rain CH0805_28 11 May 2008 3–4 May 2008 À43.185922° À71.657008° 90.0 CH0805_29 8 May 2008 6–7 May 2008 À42.178283° À71.147133° 144.3 CH0805_30 13 May 2008 3–4 May 2008 À43.541622° À71.468675° 124.1 CH0805_31 3 May 2008 À45.867920° À67.500000° 530.0 CH0805_32 6 May 2008 2 May 2008 À41.558611° À69.876944° 182.0 CH0805_33 8 May 2008 2 May 2008 À43.076944° À71.462778° 239.6 CH0805_34 8 May 2008 2 May 2008 À43.139167° À71.567500° 313.2

2À À À ﬁltered leachates were analysed for SO4 ,Cl , and F by ion chro- matography (IC), and for Si, Al, Fe, Mg, Ca, Na and K by Inductively Coupled Optical Emission Spectrometry (ICP-OES). Trace element abundances of Cr, Co, Ni, Cu, Zn, As, Se, Sr, Ag, Cd and Pb were determined by Inductively Coupled Mass Spectrometry (ICP-MS), using a three point calibration with standard solutions. Instrumen- tal drift in ICP-OES and ICP-MS results was corrected through peri- odic measurement against reference sample solutions. X-ray Photoelectron Spectroscopy (XPS) was used to measure the elemental chemistry (composition and speciation) of the uppermost 2–10 nm thick surface layer on selected ash samples (CH0805_33 and CH0805_34). The XPS analyses were carried out at the Leeds EPSCR Nanoscience and Nanotechnology Facility, using a VG Esca- lab 250 instrument with an Al Ka (1486.6 eV) X-ray source.

3. Results

3.1. Satellite observations of the Chaiten clouds

Fig. 1 shows an interrupted time series of fine ash mass for nine Fig. 1. Mass of fine ash in the atmosphere as a function of time for nine MODIS MODIS images analysed for 3–7 May. The total mass of airborne images. The time series is interrupted because of lack of satellite data coverage and ash ranged from about 0.2–0.4 Tg, which decreased with time, pre- by the sampling rate (up to four times per day). sumably due to losses from ash fallout and dispersion. The spatial distribution of ash mass loading for consecutive MODIS imagery indicates that the eruption cloud was first dispersed to the south- Fig. 3 we show two images per day (on some days there are as east on 3 May, and then to the east from 5 May (Fig. 2). The area of many as four images per day), and there is a high degree of consis- change (which we attribute to ash fallout) is shown for a series of tency between images obtained on the same day, except at the images starting on 2 May 2010 and ending on 12 May 2010 (Fig. 3). start of the eruption on 2 May 2010. The pattern of ash fallout The scale is relative, indicating the number of pixels with a differ- broadly follows ash cloud trajectories, although most of the ash ent reflectance signature to that of the reference image. The pixel is concentrated in a band to the east of the volcano, with a strong count is cumulative, but we make no attempt to relate this in a branch to the northeast. To illustrate the correspondence between quantitative manner to the build-up of ash on the surface. In the ash fallout pattern and the fine ash in the atmosphere, the fine

Fig. 2. Fine ash mass loadings retrieved from MODIS infrared satellite measurements for (a) 3 May, 05:19 UTC, (b) 5 May, 03:15 UTC, (c) 5 May, 04:55 UTC and (d) 5 May, 14:20 UTC.

ash mass loading is overlayed onto the areal ash fallout estimate 3.2. Sedimentological characteristics of the fall deposit for 5 and 8 May 2010 (Fig. 4). Volcanic ash retrievals (Fig. 2) clearly show that the ash cloud Scanning electron micrographic images (Figs. 7 and 8) indicate dispersed to the southeast on 3 May and changed to a more east- Chaitén ash fallout consisted almost entirely of non-vesicular rhy- erly dispersal between 0455 UTC and 1420 UTC on 5 May. This la- olitic glass, characterised by angular shard-like morphologies, and ter northwards migration in cloud trajectory is followed by a slight a subordinate component of blocky pyroclasts; all particles had a increase in measured surface reflectivity at 1830 UTC on 5 May and sugary appearance due to coatings of very small particles. Overall 1505 UTC on 6 May (Fig. 3) from ash fallout in the vicinity of Tre- ash fallout from the Chaitén eruption was poorly sorted and con- lew and Rawson near the E coast of Argentina. The dispersal of the sisted of particles that varied in diameter by more than two orders deposit from Phase 3 followed a northeast-trending distribution of magnitude (1 to >100 lm or about 3.25 to 10Ü); even at close and was first observed in reflectivity measurements at 1915 UTC proximity to the volcano the deposit was remarkably fine grained on 6 May as a lobe developed out towards Maquinchao, Argentina and did not vary markedly with distance (Fig. 9). For the 2–5 May (41.25°S 68.70°W). Reflectivity changes indicate that activity from 2008 phase, the mean particle size of the deposit remained fairly 7 May up to 12 May continued to build an ENE-trending deposit to- consistent at 32 lm±5lm between 95 and 530 km distance from wards San Antonio and Mar Del Plata on the Argentine coast the volcano. In contrast, for the 6–7 May 2008 phase, the deposit (40.74°S 64.97°W), that backed round to the NW as far as Concep- was coarser and mean particle size was 90 lm ± 109 lm between cion, Chile (Figs. 3 and 4). 98 and 144 km from the volcano.

Fig. 3. Areal estimates of ash fallout on the ground based on MODIS visible reﬂectances. Each panel shows analyses from a single MODIS image (two per day are shown), with the colours representing the cumulative count of pixels deemed to have changed above a threshold based on a reference image obtained prior to the eruption. The scale is a relative measure with red representing more pixels affected and purple less pixels affected. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

Fig. 4. Fine ash in the atmosphere (colour scale on right side) and an estimate of ash fallout based on MODIS visible reﬂectance (mauve) for (a) 5 May and (b) 8 May. The grey- coloured region is an area where the algorithm to estimate reﬂectance change failed due to the nature of the land surface (large topographic variability).

Fig. 5. Top left: thin ash deposit from the May 2 ash fall at El Hoyo (42.075°S 71.510°W), 12 km south from El Bolsón, Argentina, on the morning of May 3; top right: concurrent rainfall and ash fallout on 4 May 2008 (photograph Valeria Outes); bottom left: ash layer at Futaleufú border control post in Argentina (sample CH805_23) on 4 May – note the nature of the contact with white ash directly overlying soil (photograph G. Villarosa); bottom right: View of the ash deposit in Tecka, 75 km SE from Esquel, on May 4 (photograph G. Costa).

Particle size subpopulations were identiﬁed in fallout from the 3.3. Soluble element chemistry eruptions (Fig. 10): there were a total of ﬁve subpopulations at distances <250 km from the volcano and only three subpopulations The pH and major and trace element concentrations in the ash were present in fallout >500 km in fairly consistent mass propor- leachates are given in Tables 3 and 4. The charge balance was less tions. The main distance changes to note were the disappearance than 15% in all but two samples (CH0805_24 and CH0805_30), of coarse mode ash, while prominent smaller modes persisted at and the pH values were consistently close to neutral. The major greater distance in fairly consistent mass fractions. elements in the fresh ash leachates were Cl (1.5–9.7 mmol/kg),

- 72° - 70° - 68° - 66° - 64°

- 40° km - 40° 0 100 200

00.1.1

14 00.1.1 2 1 3 00.1.1 27 2266 8 00.5.5 - 42° 00.2.2 - 42° 1100 1919 2929 3 2 2244 CCHAITENHAITEN 4,5,6,7,204,5,6,7,20

1155 1100 3 1 5 3 9 3300 1177 2 - 44° 18 00.1.1 - 44° 1

00.5.5

21313 1010 15 1155 2 12 00.2.2 15 22 23 116,25 28 21 5 km 11 0 100 200 00.1.1 31 - 46° - 46°

Fig. 6. Isopach map of fallout from the 2008 Chaitén eruption courtesy of S. Watt. The isopach contours are in mm from Watt et al. (2009); the small black dots are measurement locations in the Watt et al. study. Sample locations reported in this study are shown in red and are identiﬁed by the last two digits of the sample ID detailed in Table 2. The map inset shows detail of the sample cluster 100 km from the volcano. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

followed by Na (0.9–4.1 mmol/kg), Ca (0.6–4.4 mmol/kg) and S 4. Discussion (0.1–2.9 mmol/kg). Fluoride occurred at concentrations ranging from 0.2 to 1.5 mmol/kg. The most abundant trace elements were 4.1. Particle emissions from Chaitén in the context of the global Zn, Cu and As (0.2–15 lmol/kg), while Cr, Co, Ni, Se, Sr, Ag, Cd and atmospheric silicate particulate cycle Pb were found in significantly lower concentrations. Dissolved Fe amounted to 10À4–2.7 Â 10À2 lmol/kg. Leachate compositions The global annual ash particle flux to the atmosphere is poorly of the fresh ash samples were not distinguishable from those of quantified and previous estimates are generally based on historic the ash specimens which had been exposed to light rain or for eruption records and proxies for volcanic ash emission. The pro- which no information was available (Tables 3 and 4). The total dis- portion of fine ash generated during explosive volcanic eruptions solved solid concentration in the ash leachates averaged to varies according to eruptive style (Mastin et al., 2009; Rose and 13.5 ± 4.4 mmol/kg. Further, we did not observe a trend in the Durant, 2009), and is primarily a function of magma viscosity, vol- ash leachate chemistry with distance from the source, and the atile content and open porosity (e.g., Kueppers et al., 2006). Inter- most distal ash sample (CH0805_31) produced a leachate composi- action with external water increases the proportion of fine tion comparable to samples collected closer to the volcano. In addi- particles (Mastin, 2007; Wohletz, 1983), in addition to comminu- tion to the major rock-forming elements, Cl and F (but not S) were tion and elutriation in pyroclastic flows (Dartevelle et al., 2002; detected by XPS in the surface of CH0805_33 and CH0805_34 ash Horwell et al., 2001), and brittle–ductile fracturing during extru- materials. sion of magma bodies (Tuffen et al., 2008). The blocky pyroclasts

Fig. 7. SEM images demonstrating the poor sorting of ash fallout at Esquel, 109 km E of Chaitén on 2 May 2008. Note larger mode at 300 lm diameter pyroclasts and ﬁne mode at 20–40 lm. All pyroclasts have sharp angular edges and very ﬁne particles adhere to larger ones.

observed in SEM imagery were probably formed during interaction 4.2. Correlation of ash emission and observed fallout of the magma with external water: the 2008 eruption took place in a small caldera that contained at least two small water bodies next At any moment during the period 3–7 May 2008 there was be- to the dome, and the volcano is located in a region of intense pre- tween 0.2 and 0.4 Tg of airborne ash <30 lm diameter suspended cipitation (>3000 mm/year). in the atmosphere (Fig. 1). MODIS retrievals indicated the cloud Volcanic ash emissions are episodic events that contribute contained an abundance of ﬁne ash with particle effective radii approximately an order of magnitude less silicate particles to spanning 1 lm

2 Particle size (phi) 1

-1 0 100 200 300 400 500 600 Distance (km)

Fig. 10. Particle size subpopulations identiﬁed as a function of distance from the volcano. Distal fallout (500 km) consisted of three particle size subpopulations and an additional coarse subpopulation was present in fallout 80–250 km from the volcano.

during transport (the duration of which is related to emplacement height and the wind field), and particle size selective processes act- ing during sedimentation (e.g., aggregation). Importantly, this kind of analysis demonstrates how ash deposits may be composed of fallout from many different phases of a given eruption, so care must be taken when using measurements of discrete fallout samples to reconstruct eruptions source parameters, such as initial particle size distribution, and during interpretation of polymodal particle size distributions. Therefore there is a real need for time- resolved sampling of ash fallout in order to accurately reconstruct eruption source parameters. Ash winnowing has been interpreted by many investigations of Fig. 8. SEM images of ash aggregates after breaking up on impact at locations distal ash fallout as a process which redistributes ash substan- CH805_23 (above) and CH805_24 (below) (images courtesy of Valeria Outes at Centro Atómico Bariloche facility). tially and modifies sedimentological characteristics over periods as long as months or years after deposition (e.g., Rose et al., 2003; Scasso et al., 1994). The Watt et al. samples were collected sampling at Esquel, Argentina (CH0805_4, CH0805_5, CH0805_6 weeks after deposition and do show significant but variable parti- and CH0805_7, collected on 2, 4, 6, and 9 May, respectively). The cle size differences from our samples which were collected before variation in particle size characteristics between sequential fallout, winnowing (e.g., Fig. 11). These differences will be the focus of a and between bulk deposits formed by multiple phases, is related to future study dedicated to winnowing and we use only the variability in eruption source parameters (such as eruption inten- ‘‘unwinnowed’’ materials to look for particle size differences in sity which affects the initial particle size distribution), sorting this study.

Fig. 9. Particle size as a function of distance from the volcano. The data are separated into Phases 1–2 (2–5 May) and Phases 3–4 (6–7 May). Particle size data from the 18 May 1980 eruption of Mount St. Helens are also shown.

Please cite this article in press as: Durant, A.J., et al. Long-range volcanic ash transport and fallout during the 2008 eruption of Chaitén volcano, Chile. J. Phys. Chem. Earth (2011), doi:10.1016/j.pce.2011.09.004 laect hsatcei rs s uat .. ta.Ln-ag ocncahtasotadflotdrn h 08euto fCatnvlao C volcano, Chaitén of eruption 2008 the during fallout and transport ash volcanic Long-range al. et doi: A.J., (2011), Durant, Earth as: Chem. press Phys. in article this cite Please

Table 3 Concentrations of major, minor and trace elements removed from the Chaitén ash by deionized water leaching solution.

Sample ID Si Al Mg Ca Na K F Cl SO 4 S/Cl Cl/F Cr Mn Fe CO Ni Cu Zn As Se Ag Cd Pb À1 À1 10.1016/j.pce.2011.09.004 mmol kg ash lmol kg ash Phase 1 CH0805_01 0.61 0.16 0.59 2.07 3.28 0.35 0.97 7.10 1.44 0.20 7.28 0.004 49.54 15.91 0.024 0.064 0.66 5.455 2.26 0.055 0.161 0.003

*na.:Sample not analysed; may include d.l.: 5–6 detection May ashfall limit. ie J. hile. 11 12 A.J. Durant et al. / Physics and Chemistry of the Earth xxx (2011) xxx–xxx

Table 4 aggregates broke up on deposition and measured between 0.5 Mass element ratios in the Chaitén ash leachates measured in this study, and as and 2 mm diameter. Ash fallout was also accompanied by several reported in Martin et al. (2009). brief periods of precipitation during the first week of the eruption, Mass ratio This study Martin et al. (2009) and in some cases the ash-fall was wet, as was the case of some of K:Cl 0.09 0.65 the samples collected on 4 May 2008 (Table 2). Cu:Cl 2 Â 10À4 2 Â 10À4 The May 2008 Chaitén deposit was extremely fine grained even Cu:Ni 8.8 0.5 at distances 100 km from the volcano. While there was greater Cu:As 0.12 0.07 variability in mean particle size closer to source, the lower end of Cu:Cd 193 117 Cu:Zn 0.23 0.04 the range of mean particle size converged on a value between 20 Cu:Fe 0.10 0.06 and 40 microns at most distances up to >500 km. The data are also Cu:Mn 0.03 0.12 compared to analysis of ash fallout from the Mount St. Helens 18 May 1980 eruption (Fig. 9) which was analysed using the same instrument (Durant et al., 2009). This earlier eruption injected vol- Bolson canic particulates high into the upper troposphere/lower strato- 35 sphere and transport was dominantly in the westerly jet to the Unit A (Watt et al. 2009) east of the volcano. In both cases, beyond some distance 30 (100s km from the volcano), mean particle size became remarkably Unit B (Watt et al. 2009) consistent and invariant. 25 CH0805_8 There were five particle size subpopulations identified in May CH0805_26 2008 Chaitén fallout between 80 and 250 km from the volcano, 20 CH0805_27 which decreased to three fine subpopulations between 250 and

Vol.% Vol.% 15 500 km from the volcano (Fig. 10). A similar pattern in the number and abundance of particle size subpopulations was observed in the 10 Mount St. Helens distal ash deposit. There were abundant observations of particle aggregate fallout from the Mount St. Helens erup- 5 tion cloud and the ﬁne, invariant, mean particle size, and particle size subpopulations in fallout were linked to the aggregation pro- 0 cess (Durant et al., 2009). The additional coarse subpopulation

9.50 9.00 8.50 8.00 7.50 7.00 6.50 6.00 5.50 5.00 4.50 4.00 3.50 3.00 2.50 2.00 1.50 1.00 0.50 0.00 present closer to the volcano represents single particle settling. 10.50 10.00 Particle size (phi) Based on observations of aggregate fallout and the similarity of the sedimentological characteristics, aggregation clearly played Esquel an important role in removal of ﬁne ash erupted during the Chaitén 14 eruption and in the spatial distribution of the resulting ash deposit. 02-May-08 12 4.4. Chemical leachates from eruption fallout and environmental 04-May-08 implications 10 06-May-08

09-May-08 The 2008 eruption of Chaitén presented an opportunity to study 8 transport of chemical species to local environments by rhyolitic ash fallout. It is generally accepted that experimentally-derived Vol.% Vol.% 6 ash leachate compositions reﬂect the dissolution of water-soluble compounds, including sulphate, halide salts and acids adsorbed 4 at the surfaces of the ash particles (Hinkley and Smith, 1982; Rose,

2 1977). The origin of these adsorbed compounds is ascribed to gas/ aerosol-ash interaction within the volcanic eruption plume, and la-

0 ter within the volcanic cloud (Delmelle et al., 2007; Óskarsson, 1980; Rose, 1977). The near-neutral pH values and the relatively 9.50 9.00 8.50 8.00 7.50 7.00 6.50 6.00 5.50 5.00 4.50 4.00 3.50 3.00 2.50 2.00 1.50 1.00 0.50

10.50 10.00 good charge balance of the experimentally-derived leachates indi- Particle size (phi) cate minimal quantities of acids were adsorbed onto ash particle surfaces. Fig. 11. Top: There were at least three fall units identified at El Bolson, Argentina The precise composition of the surface salts on ash is not di- (CH0805_26, CH0805_27, and CH0805_8, collected on 6, 7, and 8 May, respectively). These are compared to samples collected by Watt et al. (2009) in the same vicinity; rectly measured but previous studies inferred stochiometries in bottom: there were also at least four discrete fall units identified through ash leachates consistent with the dissolution of CaSO4 and NaCl systematic sequential sampling at Esquel, Argentina (CH0805_4, CH0805_5, deposits (de Hoog et al., 2001; de Moor et al., 2005; Hinkley and CH0805_6 and CH0805_7, collected on 2, 4, 6, and 9 May, respectively). Smith, 1982). Since Ca was present in excess of SO4 in most of the leachates (Fig. 12a), it is likely that additional soluble Ca-bear- ing salts were present at the surface of the Chaitén ash. Sodium 4.3. Propensity of ash aggregation through sedimentological may be dominantly associated with Cl, but the ClÀ surplus points fingerprinting towards the existence of other chloride salts (Fig. 12b). XPS results provide further insights into the composition of the surface salts.

Fine ash was observed to form aggregates during most sedi- The binding energy measured from the XPS Cl2p (198.6 eV) peak mentation events that resembled ‘‘snowﬂakes’’ (particle clusters, in CH0805_33 and CH0805_34 is consistent with Cl in NaCl, KCl,

PC1; Brown et al., this issue). Ash particle clusters were identiﬁed FeCl2 and/or CaCl2 (Wagner et al., 2010). Similarly, the XPS F1s peak in fallout in samples CH0805_24 and CH0805_23 (Fig. 8), and small (685.6 eV) may reﬂect F in NaF, CaF2, KF, FeF2 and/or FeF3. Finally, pellets were observed in situ in several other deposits. The size of a plot of the surface versus bulk composition for CH0805_33 and individual clusters was challenging to determine as individual CH0805_34 revealed surface enrichments in Ca, Na and Fe

Fig. 13. Surface versus bulk compositions (normalised to Si) of the CH0805_32 and CH0805_33 ash samples from Chaitén.

Fig. 12. Water-soluble Ca versus water-soluble S (A), and water-soluble Na versus water-soluble Cl (B) of the Chaitén ash leachates. See Table 3 for dataset.

(Fig. 13), suggesting the presence of coatings of mixed Ca-, Na- and Fe-rich salts. The origin of high Al concentrations measured at the surface of CH0805_32 is not immediately clear. When compared to the average leachate composition of the distal ash from the 1980 eruption of Mount St. Helens (Hinkley and Smith, 1982), Chaitén ash leachates were about 4–6 times less concentrated. This may be attributed to a range of volcanic and environmental factors including contact duration between the ash and the eruptive gas phase, ash-to-gas ratio, ash composition, Fig. 14. Relative concentrations of water-soluble F, Cl and S of the ash leachates plume humidity (Óskarsson, 1980; Rose, 1977). We also speculate from Chaitén (circles). See Table 3 for dataset. Average compositions of ash that higher leachate concentrates may result from higher ash par- leachates from the May 1980 eruption of Mount St. Helens volcano (Hinkley and Smith, 1982) and from the August 1982 eruption of Galunggung volcano (de Hoog ticle specific surface area (SSA), although this quantity was chal- et al., 2001) are shown for comparison. lenging to measure using gas adsorption as ash sample masses were extremely small and the corresponding measurement errors large. Several authors have used the S:Cl ratio measured in ash leach- May 2008 phases did not produce distinguishable S:Cl and Cl:F ra- ates to infer the composition of the eruptive gas phase. Except in tios (Table 3). one sample, the molar S:Cl ratio in the Chaitén ash leachates was Based on field examination of the ash-fall deposits over Argen- consistently less than one, a value lower than that found for ash tina, Watt et al. (2009) estimated that the total mass of ash erupted from other explosive magmatic eruptions, such as Mount St. He- during the first of week of Chaitén explosive eruption in May 2008 lens 1980 and Galunggung (Indonesia) 1982 eruptions (Fig. 14). was 1.7 Â 1011 kg. Assuming that this value represents a mini- For comparison, high temperature magmatic gases associated with mum mass of magma erupted over this period, and knowing that subduction zones typically display a S:Cl ratio in the range 2–8 the pre-eruptive magmatic S content was less that 50 mg S kgÀ1 (Giggenbach, 1996). Thus, the low S:Cl ratio values of the Chaitén (Lowenstern et al., 2010), the total mass of S which may have been ash leachates may reflect a relatively S-poor eruptive gas composi- released during explosive degassing was 8.5 Â 106 kg. Using an tion, in agreement with satellite observations (Carn et al., 2009)as average S concentration in the ash leachate of 1.1 mmol kgÀ1 of well as with recent petrological estimates (Lowenstern et al., ash (or 34 mg S kgÀ1 of ash), the total amount of S scavenged 2010). Similarly, the high Cl:F ratio values obtained for the ash and returned to the Earth’s surface via ash deposition was leachates may suggest gas–ash interaction within a Cl-rich plume. estimated to 5.8 Â 106 kg (or 68% of the S initially released) Of note, the ash materials erupted during the 2–5 May and 6–9 with the rest of the S (2.7 Â 106 kg) remaining in the gaseous

Please cite this article in press as: Durant, A.J., et al. Long-range volcanic ash transport and fallout during the 2008 eruption of Chaitén volcano, Chile. J. Phys. Chem. Earth (2011), doi:10.1016/j.pce.2011.09.004 14 A.J. Durant et al. / Physics and Chemistry of the Earth xxx (2011) xxx–xxx and aerosol phases. According to satellite measurements, These points are significant when considering the impact of ash 5 Â 106 kg of S were emitted in Chaitén eruptive plumes on 2, 6 deposition on ocean biogeochemical cycles. Our results indicate and 8 May 2008 (Carn et al., 2009). This figure is in reasonable that the low Fe concentration measured in the ash leachate is un- agreement with that derived above, but a perfect match between likely to be a good indicator for inferring the potential of ash to act the calculated and observed quantities of S injected into the atmo- as a source of soluble Fe in the ocean. First, the amount of Fe sphere is obtained by using a lower average S concentration (i.e., needed to enhance phytoplankton is low anyway (see Duggen 0.65 mmol kgÀ1 of ash) for the ash leachate. In this case, scaveng- et al. 2009). Second, the ash leachate method is not designed to ing by ash would account for 42% of the magmatic S release, a reproduce the seawater environment (see Duggen et al. (2009) fraction comparable to those derived for explosive eruptions else- and Olgun et al. (in press)). Iron is poorly soluble at near-neutral where (de Hoog et al., 2001; de Moor et al., 2005; Rose, 1977; pH values (such as those observed in the experimentally-derived Varekamp et al., 1984). ash leachates) and dissolved Fe concentration is controlled by Fe Deposition of volcanic ash onto terrestrial and aquatic ecosys- (oxy)hydroxide precipitation (Stumm and Furrer, 1987). Thus tems may lead to various physical, chemical and biological effects there is high potential for Fe release following wet deposition of (Witham et al., 2005; and references therein). Data collected in ash over the ocean. Therefore, we argue that the Fe release poten- June 2008 and January 2009 in the area impacted by Chaitén ash tial of the Chaitén ash should not be dismissed solely on the basis led Martin et al. (2009) to argue that ephemeral lakes which re- of its total Fe content (e.g., as suggested by Watt et al., 2009). ceived more than 2 mm of ash were subject to compositional changes. This suggestion was based on the positive correlations be- 5. Conclusions tween the concentrations of certain elements (Rb, Tl, K, As, Cs, Mn, Zn, Cl, Cd, Fe, B, Cu and Ni) and thickness of ash deposited in the MODIS infrared ash particle retrievals and surface reflectance five lakes surveyed. The underlying assumption was that Ca served measurements show good spatiotemporal correspondence be- as a reference element to account for the differences in dilution between observed cloud trajectories and increased surface reflec- tween the lakes; that is, the lake Ca concentration was not influ- tance inferred to indicate ash fallout. At any one moment during enced by the entry of ash. This hypothesis is however challenged the period 3–7 May 2008 there was between 0.2 and 0.4 Tg of air- by our ash leachate measurements (Table 3). By considering these borne ash <30 lm diameter suspended in the atmosphere. The ash along with the ash thickness and lake morphometric data pre- deposit formed by the Chaitén eruption was dominated by fine sented in Martin et al. (2009), we calculated that three ephemeral grained and very fine grained angular rhyolitic glass shard particles lakes probably received an input of ash-derived Ca corresponding up to a distance >500 km from the volcano. Particle size varied very to at least 20% of the total dissolved Ca in the lake water. Martin little beyond a distance of about 300 km, and the extent of ash-fall et al. also derived elemental ratios in the soluble material adhering continued and thinned to near zero to reach the east coast of the to the ash based on the lake chemical analyses (Table 3 in Martin continent. It is inferred that fallout of fine ash from the Chaitén et al. (2009)). However, comparison of these values with those eruption cloud was dominantly driven by the formation of aggre- measured in the ash leachates (Table 4) revealed significant dis- gates, perhaps aided by association with icy hydrometeors. Bulk crepancies, possibly attributable to higher lake K, Ni, Zn, Fe and ash leachate and XPS analyses revealed surface enrichments in Mn concentrations that could be generated from ash leaching only. Ca, Na and Fe, which suggests the presence of coatings of mixed This suggests that beside ash deposition, other environmental fac- Ca-, Na- and Fe-rich salts on ash particle surfaces prior to deposi- tors need to be considered in order to explain the reported variabil- tion. The low S:Cl ratios in Chaitén ash leachates may reflect a rel- ity in the ephemeral lake compositions. atively S-poor eruptive gas composition, and high Cl:F ratios The lake locations reported in Martin et al. were revisited as part suggest gas–ash interaction within a Cl-rich plume. It is estimated of the current study and it was noted that whereas the water bodies that ash fallout had potential to scavenge 42% of total S released in Martin et al. were reported as ‘‘ephemeral lakes’’, a number of into the atmosphere by the eruption. Furthermore, our results these water bodies were in fact ‘‘mallines’’ (wetlands) which are suggest that there may be a pool of Fe at the surface of ash particle surface expressions of the local subsurface hydrological system. surfaces that becomes available after exposure to low pH environ- Usually these features develop in areas of active drainage systems ments typical of volcanic plumes and clouds. It is possible, there- with low relief and are linked closely to the water table. As there fore, that the Fe release potential of Chaitén ash fallout over are fluxes in and out of these water bodies via the groundwater sys- oceans could have been high enough to effect ocean productivity. tem, complications arise because the original water composition is It follows that bulk leachate analyses are unlikely to be a good indi- not known. It is therefore challenging to assess the impact of leach- cator of the potential of ash to act as a source of soluble Fe in the ates from ash fallout without information on the original water ocean. chemistry of a given water body, which would require a long time series of measurements to characterise any natural variability. XPS analyses revealed that the ash surfaces of CH0805_33 and Acknowledgements CH0805_34 were strongly enriched in Fe (Fig. 13). This result con- trasts with the systematic depletion of surface Fe reported previ- AJD gratefully acknowledges support from the Natural Environ- ously in various ash specimens (Delmelle et al., 2007), although mental Research Council. Field work was supported by a CONICET the reason for this difference has not yet been established. The high special grant. We especially thank the Administración de Parques value of Fe in the ash surface is not reflected in the leachate anal- Nacionales, INTA, Municipalidad de Esquel, Dirección de Bosques yses; that is, the concentrations compare to those observed in de Chubut for their participation in the ash collection network. other ash leachate studies (Witham et al., 2005). However, recent We also thank Valeria Outes at Centro Atómico Bariloche facility detailed measurements of the potential for Fe release in seawater for providing SEM images of ash fallout. Sebastien Watt, Tamsin for various ash samples indicate above-average values for the Cha- Mather and David Pyle are acknowledged and thanked for gener- itén ash (Olgun et al., in press), which may reflect the Fe enrich- ously allowing access to their data, discussion on the sedimentol- ment detected at the ash particle surfaces. Furthermore, Fe at the ogy of the deposit, and assistance generating the fallout map. ash surface may become readily mobilised on exposure to low Marcelo Marquez and Nilda Menegatti from Universidad de la Pat- pH values, for example when the ash is subject to cloud processing agonia San Juan Bosco supplied key distal samples and Claire Hor- in the atmosphere (see Ayris and Delmelle, this issue). well kindly supplied three ash samples.

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